1. Field of the Invention
The present invention relates to an air bearing and more particularly to an air bearing device for a dental handpiece.
2. Prior Art
The structure of an existing air journal type handpiece has several problems.
As seen from FIG. 1, a front sectional view, a turbine rotor 2 holding a cutting tool shaft 1 is positioned in a bearing so as to form therebetween a radial gap 3 and thrust gaps 4. High velocity compressed fluid (compressed air) is fed into these gaps 3 and 4 through air channels 6, inlet holes 7, and bearing orifices 8. The orifices 8 form a thin air-film in the gaps 3 and 4 for the rotating rotor 2 (at 500,000 r.p.m.max.) to be journaled radially as well as thrustwise. In the figure, there are also shown an exhaust channel 9, turbine blades 10, elastic --O--rings 11, air channel 12 for driving the turbine blades 10, handpiece casing 13, and an end cap 14. In the prior art the bearing 5 includes the surface of a rotatable body that has a tool shaft 1 as its axis such as a hollow cylinder or the like or a truncated cone. However, with the combination of such a rotatble body and a rotor 2, there is a limit to the load capacity or bearing stiffness. The small bearing stiffness causes a so-called whirl phenomenon, and the small load capacity causes contact noise between the bearing 5 and the rotor 2 to aggravate the clinic enviornment. In addition, the design damages the confronting faces of the rotor 2 and the bearing 5, resulting in poor durability. Furthermore, it is noted among others that the rotor speed decreases as much as 1,000,000 r.p.m. due to the whirl phenomenon compared with when the cutting tool shaft 1 is not set in the rotor.
The present inventors secured earlier two patents that solve the problems in the prior art: U.S. Pat. Nos. 4,326,846 and 4,209,293. The former patent relates to an invention in which each of the turbine blades is provided with a partially communicating channel, and the part of the air guided by the channel drives the turbine blade in its rotational direction to improve the torque efficiency. The latter patent is for an invention in which throttling parts are provided in the thrust end face of the air bearing in order to substantially increase the centripetal thrust gap to improve the bearing stiffness, and the throttling parts defines an air pool.